People that I have talked to, that have used bed sprays as an exterior paint, say that over time they get dirty and are hard to clean. I knew of one person who used a clear coat over a bed liner with good results. Hammerite might be a good choice. It is textured and has a hard finish.
I am bouncing around the idea of painting the lower half of my Legacy. It has quit a few rock dings but no rust to speak of.